A siloxane is any chemical compound composed of units of the form R2SiO, where R is a hydrogen atom or a hydrocarbon group. They belong to the wider class of organosilicon compounds.
Siloxanes can have branched or unbranched backbones consisting of alternating silicon and oxygen atoms -Si-O-Si-O- (connected by a σ-bond with a length of 1,64 ± 0,03 Å[1]), with side chains R attached to the silicon atoms.[2] More complicated structures are also known, for example eight silicon atoms at the corners of a cube, connected by 12 oxygen atoms as the cube edges.[3]
The word siloxane is derived from the words silicon, oxygen, and alkane.
Polymerized siloxanes with organic side chains (R ≠ H) are commonly known as silicones or as polysiloxanes. Representative examples are [SiO(CH3)2]n (polydimethylsiloxane) and [SiO(C6H5)2]n (polydiphenylsiloxane). These compounds can be viewed as a hybrid of both organic and inorganic compounds. The organic side chains confer hydrophobic properties while the -Si-O-Si-O- backbone is purely inorganic. Also halogens, mainly chlorine, are possible as side compound R.[4]
Contents |
M-units: (CH3)3SiO½, D-units: (CH3)2SiO, T-units: (CH3)SiO2
| Cyclic siloxanes | Linear siloxanes |
|---|---|
| D3: hexamethylcyclotrisiloxane | MM: hexamethyldisiloxane |
| D4: octamethylcyclotetrasiloxane | MDM: octamethyltrisiloxane |
| D5: decamethylcyclopentasiloxane | MD2M: decamethyltetrasiloxane |
| D6: dodecamethylcyclohexasiloxane | MDnM: polydimethylsiloxane |
Industrially, siloxanes are fabricated [5]
Siloxanes can be found in products such as cosmetics, deodorant, defoamers, water repelling windshield coating, lubricants, molded lenses for high-powered LED's[6],food additives and some soaps. They occur in landfill gas and are being evaluated as alternatives to perchloroethylene for drycleaning. Perchloroethylene is widely considered environmentally undesirable.
In internal combustion engines, siloxanes are oxidized to silicon dioxide which then forms deposits on pistons and cylinder heads which are extremely abrasive and cause damage to the internal components of the engine. Engines can require a complete overhaul at 5,000 h or less of operation.[7][8] Deposits on the turbine of the turbocharger will eventually reduce the components efficiency.
Siloxanes can be a serious issue for some materials, used in the membrane technology, like polyvinylidene fluoride e.g.. Due to their hydrophobe behaviour and their large molecular weight, an accumulation of siloxanes on the membrane surface is possible, resulting in fouling.